Yes, all feminine nouns that I can think of can be used both as masculine and feminine.

I'm not sure if it is a general rule, however, and for example jenten is only common in the Bergen area, as far as I know. The same applies to for example tante (aunt), where tanten sounds very strange to people from Østlandet. (But bestemoren (the grandmother) is perfectly fine. Yup, it makes no sense.)

I recommend sticking to the ei jente / jenta form, as it's the simplest to remember and you don't need to learn the regional exceptions.

I can't think of any examples of nouns in intetkjønn that can have multiple genders in singular, but they can often switch in plural. For example husene / husa (the houses).

My recommendation is to mostly ignore all of this. Genders and prepositions are always hard when learning a new language, and are pretty much arbitrary anyway :-)

Edit: I asked a friend of mine from Bergen. She said "We don't have feminine nouns in Bergen."